Is is necessary to braise hamburger before adding it as an ingredient in a "slow cooked" soup?

Braise? No. Brown? Not necessary, but as nance points out, it has it's upsides. As far as food safety, if your slow cooker is properly calibrated that should take care of it.

I would, if only to reduce the considerable amount of fat it would add. Browning it first would eliminate you having to skim the fat from the soup as well as add a little more flavour.